Our long term objective is to understand the biochemical mechanisms and physiological significance of estrogen receptor alpha (ERalpha) and estrogen receptor beta (ERbeta) activation of gene expression at target genes with AP-1 or CRE sites (AP-1/CRE). Such AP1/CRE target genes, including cyclin D1, lack classical estrogen response elements (EREs) but are nonetheless induced by ER action. ERa activates AP-1/CRE primarily with estrogen and ERbeta, in contrast, only with antiestrogens. To understand how ERs work at AP- 1/CRE, we will exploit a mutation that makes both ERs selectively and dramatically superactive at AP-1/CRE sites and only with estrogen. ER induction of AP-1/CRE target genes may underlie some of the most profound effects of estrogen, those on cellular proliferation in the mammary gland and reproductive track. We find that the selectively superactive ERalpha, K206A, drives overexpression of cyclin D1 and hyper-proliferation in the genital track of transgenic female mice. K206A also drives overexpression of cyclin D1 in cultured cells, but the presence of ERa inhibits such activation. We thus wish to explore the hypothesis that ERa action at AP-1/CRE elements drives some of the proliferative effects of estrogen, especially in the mammary gland and reproductive track, and that the presence of ERbeta, by inhibiting activation of AP-1/CRE targets, modulates these effects. Our aims are: Aim 1. Determine the molecular mechanisms of ERalpha-estrogen activation and K206A super-activation at AP-1/CRE elements. Aim 2. Determine the physiological role of ERalpha activation of AP-1/CRE elements in the mammary gland. Aim 3. Determine the mechanism whereby ERbeta inhibits the ability of ERalpha and ERalpha K206A to activate at AP-I/CRE with estrogen. Aim 4. Determine whether ERbeta, which inhibits ERbeta stimulation of cyclin D1 gene expression, also inhibits cellular proliferation mediated by ERalpha. If successful, these studies will test the hypothesis that ERalpha action at AP-1/CRE target genes mediates proliferation, and that ERbeta modulates such action. The knowledge gained will be greatly useful for developing a new generation of drugs to prevent estrogen induced cancers.